1. Technical Field
The present invention relates to an image reading apparatus.
2. Related Art
Image reading apparatuses developed to date include those incorporated with an automatic document feeder (hereinafter, ADF) that feeds sheets of an original one by one from a bundle of sheets of the original stacked on a tray, to a predetermined reading position. In the image reading apparatus with the ADF, when a paper edge detection sensor located upstream of the reading position detects a leading edge of the original sheet, a paper feed roller is rotated by an amount corresponding to the distance between the detection position and the reading position, before reading is started. Specifically, the paper edge detection sensor is located several centimeters upstream of the reading position, because of structural restriction of the image reading apparatus. Accordingly, in case that the original sheet is bent or slips on its way from the detection position to the reading position, the leading edge of the original sheet may have actually failed to reach the reading position, despite it being assumed that the leading edge of the original sheet has reached the reading position, from the viewpoint of operation control. To minimize such a drawback, for example an image reading apparatus according to JP-A-2005-57483 is configured to detect the leading edge of the original sheet immediately before the reading position, utilizing a light source and a photodetector employed for reading an image. More specifically, the image reading apparatus is configured, as shown in FIG. 8, such that the original sheet is transported onto a white reference roller 210, and that light from the light source 212 is obliquely emitted from behind to the original sheet being transported. At the moment that the leading edge of the original sheet reaches the position immediately before the reading position, the shadow of the leading edge is projected on the white reference roller 210, and detected by the photodetector 214. Then the original sheet is transported by a predetermined distance from the position where the shadow has been detected, so that the leading edge of the original sheet is led to the reading position.
In the foregoing image reading apparatus, however, the position of the leading edge of the original sheet at the time that the shadow is detected varies depending on the thickness of the original sheet. As is apparent from FIG. 8, the position of the leading edge of the original sheet at the time that the shadow is detected is different between the original sheet of a predetermined thickness (solid lines) and another original sheet thicker than the predetermined thickness (dash-dot lines). This leads to a drawback that using an original sheet having a different thickness degrades the accuracy in detecting the position of the leading edge of the original sheet utilizing the shadow of the leading edge.